Method for removing aluminide coating from metal substrate and turbine engine part so treated

ABSTRACT

A method for selectively removing an aluminide coating from at least one surface of a metal-based substrate by: (a) contacting the surface of the substrate with at least one stripping composition comprising nitric acid at a temperature less than about 20° C. to degrade the coating without damaging the substrate; and (b) removing the degraded coating without damaging the substrate. Also disclosed is a method for replacing a worn or damaged aluminide coating on at least one surface of a metal-based substrate by selectively removing the coating using the above steps, and then applying a new aluminide coating to the surface of the substrate. Turbine engine parts, such as high-pressure turbine blades, treated using the above methods are also disclosed.

BACKGROUND OF THE INVENTION

This invention relates to a method for removing an aluminide coatingfrom a metal-based substrate. More particularly, the invention isdirected to a method for selectively removing an aluminide coating byusing a stripping composition to degrade the coating and then removingit without damaging the substrate. The invention also relates to aturbine engine part having an aluminide coating, at least a portion ofwhich has been selectively removed by the above method.

A variety of coatings are often used to protect metal parts exposed tohigh temperatures, such as parts made from superalloys. For example, gasturbine engine components (and other industrial parts) are often formedof superalloys that can withstand a variety of extreme operatingconditions. Such parts are usually covered with coatings to protect themfrom environmental degradation, including the adverse effects ofcorrosion and oxidation. Coatings used on components in gas turbine hotsections, such as blades, nozzles, combustors, turbine shrouds andtransition pieces, generally belong to one of two classes: diffusioncoatings or overlay coatings.

Diffusion coatings are typically formed of aluminide-type alloys, suchas nickel-aluminide; a noble metal-aluminide such as platinum-aluminide;or nickel-platinum-aluminide. Overlay coatings typically have thecomposition MCrAl(X), where M is an element selected from the groupconsisting of Ni, Co, Fe, and combinations thereof, and X is an elementselected from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, andcombinations thereof. Diffusion coatings are formed by depositingconstituent components of the coating, and reacting those componentswith elements from the underlying substrate, to form the coating by hightemperature diffusion. In contrast, overlay coatings are generallydeposited intact, without reaction with the underlying substrate.

During service, diffusion and overlay coatings are often exposed tooxidative conditions. For example, coatings on turbine airfoils aretypically subjected to oxidation in the hot gas path during normaloperation. Under such conditions, with temperatures in the range ofabout 525-1150° C., various oxidative products are formed on thecoatings. For example, aluminum oxides and other metal oxides, includingnickel oxide, cobalt oxide, chromium oxide, and other base metal oxides,often form on simple aluminide and platinum-aluminide coatings. Aluminumoxides, chromium oxides, and various spinels often form on theMCrAl(X)-type coatings.

When turbine engine components are overhauled, the protective coatingsare often removed to allow inspection and repair of the underlyingsubstrate. Various stripping compositions have been used to remove thecoatings. Usually, the oxide materials must be removed before thecoatings can be treated with the stripping composition. Varioustechniques have been used for oxide removal. For example, oxidematerials often have been removed from external sections of turbinecomponents by grit blasting.

Alternatively, turbine components have sometimes been treated in anoxide-removal solution comprising a strong mineral acid or a strongcaustic. Examples of such mineral acids are hydrochloric acid, sulfuricacid, and nitric acid. The caustic solutions usually include sodiumhydroxide, potassium hydroxide, or various molten salts. Repeatedtreatments sometimes are used to remove the oxide. After removal of theoxide is completed, the substrate is then typically immersed in anothersolution suitable for removing the coating material itself. In currentpractice, the aluminide materials are often stripped from the substrateby exposure to various acids or combinations of acids, e.g.,hydrochloric acid, nitric acid, and phosphoric acid.

There are some drawbacks associated with the use of the variousstripping compositions mentioned above. Some stripping compositions donot remove sufficient amounts of the aluminide material. Othercompositions that remove the aluminides also attack the base metal ofthe substrate, pitting the base metal or damaging the metal viaintergranular or interdendritic (in the case of single crystalmaterials) attack. Some stripping compositions are used at elevatedtemperatures, e.g., above about 75° C. to speed the reaction and removalof the coating. Operation at these temperatures can promote increasedattack of the base metal and may require masking materials to protectselected portions of the metal part, e.g., airfoil internal surfaces.Elevated temperature processes also increase energy costs andpotentially require additional safety precautions. Airfoil internalsurfaces are often filled with wax or plastic to protect surfaces thatdo not require stripping. These materials must be removed before usingthe part, adding additional manufacturing steps and cost. Moreover,conventional treatment solutions that employ large quantities of strongmineral acids may emit an excessive amount of hazardous fumes that mustbe scrubbed from ventilation exhaust systems.

Some processes use grit-blasting prior to acid treatment to pretreat andactivate the substrate surface, and after exposure to the strippingcomposition to remove residual degraded coating. These steps can be verytime-consuming, and can also damage the substrate and limit part life.Special care may need to be taken to prevent grit-blasting damage to thesubstrate or any protective coating not being removed from the metalpart. Moreover, grit-blasting cannot generally be used to remove oxidematerial from internal passages or cavities in metal parts. For example,grit-blasting would not be suitable for use in the internal coolingpassages of high pressure turbine blades where the grit particles couldblock the internal passages.

It is thus apparent that new processes for removing aluminide coatingsfrom metal substrates would be welcome in the art. It would be desirableif the processes remove substantially all of the aluminide coating,while not damaging the base metal. Moreover, it would be desirable ifthe processes could be carried out at lower temperatures to minimize oreliminate base metal attack. It would also be desirable if the processeseliminate preliminary steps like grit-blasting, so that they can be usedto effectively remove coatings from internal sections of metal partswithout blocking internal passages.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, this invention relates to a method for selectivelyremoving an aluminide coating from at least one surface of a metal-basedsubstrate, comprising the following steps:

-   -   (a) contacting the surface of the substrate with at least one        stripping composition comprising from about 20% to about 40%        nitric acid, by weight of the composition, at a temperature less        than about 20° C. for at least about 2 hours to degrade the        coating without damaging the substrate; and    -   (b) removing the degraded coating without damaging the        substrate.

As used herein, “selective removal” of the aluminide coating refers tothe removal of a relatively large percentage of the aluminide materialwhile removing only a very small portion (or none) of the substratematerial. Any affected portion of the metal substrate is usually to adepth of less than about 0.0015 inches (about 38 microns), typicallyless than about 0.001 inches (about 25 microns), and more typically lessthan about 0.0005 inches (about 13 microns).

The term “aluminide material” in this context is meant to include avariety of materials typically used in coating metal alloys (especiallysuperalloys), or which are formed during or after the coating process.Non-limiting examples include simple aluminide, platinum-modifiedaluminide, nickel aluminide, cobalt aluminide, platinum-nickelaluminide, refractory-doped aluminide, or alloys comprising one or moreof those compounds.

In another aspect, this invention relates to a method for selectivelyremoving an aluminide coating from at least one internal surface of ametal-based substrate, comprising the following steps:

-   -   (a) contacting the surface of the substrate with an aqueous        solution comprising from about 10% to about 50%, by weight of        the solution, of caustic at a temperature of from about 60° C.        to about 100° C. for from about 1 to about 4 hours;    -   (b) contacting the surface of the substrate with at least one        stripping composition comprising from about 20% to about 40%        nitric acid, by weight of the composition, at a temperature less        than about 20° C. for at least about 2 hours to degrade the        coating without damaging the substrate;    -   (c) contacting the surface of the substrate with an aqueous        solution comprising from about 10% to about 50%, by weight of        the solution, of caustic at a temperature of from about 60° C.        to about 100° C. for from about 20 minutes to about 2 hours;    -   (d) contacting the surface of the substrate with at least one        stripping composition comprising from about 20% to about 40%        nitric acid, by weight of the composition, at a temperature less        than about 20° C. for at least about 2 hours to degrade the        coating without damaging the substrate; and    -   (e) contacting the surface of the substrate with an aqueous        solution comprising from about 10% to about 50%, by weight of        the solution, of caustic at a temperature of from about 60° C.        to about 100° C. for from about 1 to about 4 hours to remove the        degraded coating without damaging the substrate.

In another aspect, the invention relates to a turbine engine part havinga metal-based substrate and an aluminide coating on at least one surfacethereof, at least a portion of which coating has been selectivelyremoved from the surface of the substrate using the above methods.

Another aspect of the invention relates to a method for selectivelyremoving an aluminide coating from at least one internal surface of asuperalloy substrate, comprising the following steps:

-   -   (a) contacting the surface of the substrate with at least one        stripping composition comprising from about 25% to about 35%        nitric acid, by weight of the composition, at a temperature of        from about 0° C. to about 15° C. for from about 4 hours to about        10 hours to degrade the coating without damaging the substrate;        and    -   (b) removing the degraded coating without damaging the        substrate.

In yet another aspect, the invention relates to a method for replacing aworn or damaged aluminide coating on at least one surface of ametal-based substrate, comprising the steps of selectively removing thealuminide coating from the surface using the above methods, and thenapplying a new aluminide coating to the surface of the substrate.

Other details regarding the various embodiments of this invention areprovided below.

DETAILED DESCRIPTION OF THE INVENTION

The above methods comprise the step of contacting the surface of thesubstrate with at least one stripping composition comprising from about20% to about 40% nitric acid, by weight of the composition, at atemperature less than about 20° C. for at least about 2 hours to degradethe coating without damaging the substrate.

Within the above ranges, various stripping compositions and processingconditions can be used in the process of the invention. The choice of aparticular composition or condition will depend on various factors, suchas the type of substrate, the type of aluminide coating being removedfrom the substrate, the intended end use for the substrate, and thepresence or absence of additional treatment steps (e.g., pretreatment,desmutting, neutralization and/or rinsing steps). Those skilled in theart will be able to choose appropriate stripping compositions andprocessing conditions for a given situation, based on the teachingsherein.

The substrate of the present invention can be any metallic material oralloy typically protected by an aluminide coating. As used herein,“metallic” refers to substrates that are primarily formed of metal ormetal alloys, but which may also include some non-metallic components.Non-limiting examples of metallic materials comprise at least oneelement selected from the group consisting of iron, cobalt, nickel,aluminum, chromium, titanium, and mixtures thereof (e.g., stainlesssteel).

Often, the substrate is a heat-resistant alloy, e.g., a nickel-basedmaterial or cobalt-based material. Such materials are described invarious references, including U.S. Pat. Nos. 5,399,313 and 4,116,723.The type of substrate can vary widely, but it is often in the form of ajet engine part, such as an airfoil component. As another example, thesubstrate may be the piston head of a diesel engine, or any othersubstrate requiring a heat-resistant or oxidation-resistant coating. Thesubstrate may also be in the form of a houseware item (e.g., cookware),or other industrial hardware or equipment.

The metallic material is often a superalloy, typically nickel-, cobalt-,or iron-based, although nickel- and cobalt-based alloys are favored forhigh-performance applications. The base element, typically nickel orcobalt, is the single greatest element in the superalloy by weight.Nickel-based superalloys usually include at least about 40% Ni, and atleast one component selected from the group consisting of cobalt,chromium, aluminum, tungsten, molybdenum, titanium, and iron.

Examples of nickel-base superalloys are designed by the trade namesInconel®, Nimonic®, and René®, and include directionally solidified andsingle crystal superalloys. Cobalt-based superalloys usually include atleast about 30% Co, and at least one component from the group consistingof nickel, chromium, aluminum, tungsten, molybdenum, titanium, and iron.Examples of cobalt-based superalloys are designated by the trade namesHaynes®, Nozzaloy®, Stellite® and Ultimet®.

The aluminide coating on the substrate may be applied in a variety of,locations on a component. In the case of a turbine engine, the coatingis often applied on combustor liners, combustor domes, shrouds,airfoils, including buckets or blades, nozzles, and vanes. The coatingcan be found on the flat areas of substrates, as well as on curved orirregular surfaces. The coating may also be formed on the surfaces ofinternal cavities in the substrates, e.g., indentations, hollow regions,or holes. For example, the cavities can be in the form of radial coolingholes or serpentine passageways, which can have an overall length of upto about 30 inches (about 76.2 cm) in turbine engine airfoils. It isoften difficult to remove the coating from the surface of these cavitiesby conventional, line-of-sight processes such as grit blasting, plasmaetching, or laser ablation.

The thickness of the coating will depend on a variety of factors. Theseinclude the length of service time for the component, its thermalhistory, and the particular composition of the coating and substrate.Usually the coating has a thickness in the range of from a few micronsto about 150 microns, and most often in the range of from about 25microns to about 75 microns.

The stripping composition of the present invention comprises from about20% to about 40%, typically from about 25% to about 35%, more typicallyfrom about 28% to about 32%, by weight of the composition, of nitricacid. This relatively high concentration of nitric acid often causesless base metal attack than lower concentrations of nitric acid. Thebalance of the stripping composition typically is a suitable solvent,such as water, although minor amounts of other acids and additives asdescribed below may be included in the composition. Inorganic acids,such as hydrochloric acid and sulfuric acids, and aliphatic and aromaticacids useful herein are disclosed in U.S. Pat. No. 5,976,265, Sangeetaet al.

The stripping composition of the present invention may include variousother additives that serve a variety of functions. Non-limiting examplesof these additives are solvents, inhibitors, dispersants, surfactants,chelating agents, wetting agents, deflocculants, stabilizers,anti-settling agents, oxidizing agents, reducing agents, and anti-foamagents. Those of ordinary skill in the art are familiar with suchadditives, and with effective levels for their use.

In certain embodiments, an organic solvent may be used to reduce theactivity and increase the wetting capability of the nitric acid relativeto the substrate. (The chemical interaction between an acid and ahydrocarbon solvent will often differ from the interaction between theacid and a solvent like water.) The combination of nitric acid and theorganic solvent may remove substantially all of the aluminide coatingmaterial without adversely affecting the substrate. As used herein,“activity” generally refers to a measurement of the reactivity of theacid toward the substrate and/or the aluminide coating being removedfrom the substrate.

Examples of organic solvents which generally meet the requirements forthis class of stripping compositions are aliphatic alcohols, aromaticalcohols, chlorinated alcohols, ketones, nitrile-based solvents,nitrated hydrocarbon solvents, nitrated aromatic solvents such asnitrobenzene, chlorinated hydrocarbons, amines, and mixtures of any ofthe foregoing. Specific examples of aliphatic alcohols useful herein aremethanol, ethanol, and isopropanol. Mixtures of alcohols may be used aswell. Specific examples of aromatic alcohols are phenols and substitutedphenols.

The use of such mixtures may occasionally result in slight pitting, orin a small amount of corrosion of the substrate, which is typicallysubstantially uniform. As used herein, “uniform corrosion” refers to theremoval of a very thin, continuous layer of the substrate, usually lessthan about 2 microns in thickness. Uniform corrosion and slight pittingare not a significant drawback for some end uses of the substrate. Thisis in contrast to the occurrence of severe “pitting”, which results inholes in the substrate, often to a depth of at least about 25 microns,and usually to a depth in the range of from about 25 microns to about500 microns.

In some embodiments, the stripping composition further includes awetting agent. The wetting agent reduces the surface tension of thecomposition, permitting better contact with the substrate and thealuminide coating, particularly on internal surfaces of metal parts, toimprove stripping of the aluminide coating. Suitable wetting agentsinclude polyalkylene glycols, glycerol, fatty acids, soaps, emulsifiers,and surfactants. The wetting agent is usually present at a level in therange of from about 0.1% by weight to about 5% by weight, based on thetotal weight of the composition.

Inhibitors such as acetic acid are sometimes employed in the strippingcomposition to lower the activity of the acid in the composition. Thelowered activity in turn decreases the potential for pitting of thesubstrate surface. If used, the level of inhibitor usually is from about1% by weight to about 15% by weight, based on the weight of thestripping composition.

Oxidizing agents are sometimes used in the stripping composition toprevent the formation of a reducing environment. Examples includeperoxides (e.g., hydrogen peroxide), chlorates, perchlorates, nitrates,permanganates, chromates, and osmates (e.g., osmium tetroxide). Thelevel of oxidizing agent used is usually from about 0.01% by weight toabout 5% by weight, based on the weight of the entire strippingcomposition.

The stripping composition may be applied to the substrate in a varietyof ways. In some embodiments, the substrate is immersed, eitherpartially or fully, in a bath of the composition. Immersion in thismanner (in any type of vessel) often permits the greatest degree ofcontact between the composition and the coating being removed. Thesubstrate may be lowered into the bath using a suitable rack (forexample, one having a polypropylene or other non-conductive surface)that can be raised to remove the substrate after the desired immersiontime is reached. Immersion time and bath temperature will depend on manyof the factors described above, such as the type of coating beingremoved and the acid (or acids) being used in the bath. However, thebath is typically maintained at a temperature below about 20° C. whilethe substrate is immersed therein. In some embodiments, the bath ismaintained at a temperature of from about 0° C. to about 15° C., oftenfrom about 4° C. to about 12° C. Temperatures much higher than 20° C.typically result in more rapid removal of the aluminide coating and maycause excessive pitting of the base metal. Use of the lower temperaturesherein protects the metal substrate and masking materials that may bepresent, and also reduces safety hazards associated withhigher-temperature baths when volatile components are present.

Baths comprising the stripping composition are often stirred orotherwise agitated while the process is carried out, to permit maximumcontact between the stripping agent and the coating being removed. Avariety of known techniques can be used for this purpose, such as usingimpellers, ultrasonic agitation, magnetic agitation, gas bubbling, orcirculation-pumps. Immersion time in the bath will vary based on many ofthe factors discussed above. On a commercial scale, the immersion timewill usually range from about 2 hours to about 20 hours in total, whichmay be split among two or more stripping steps. In some embodiments, thetotal immersion time will be from about 3 to about 15 hours, typicallyfrom about 4 to about 10 hours, more typically from about 6 to about 8hours. Longer stripping times within the above ranges promote morecomplete removal of the aluminide coating but can cause greater basemetal attack. Thus, the stripping time, the concentration of nitric acidin the stripping composition, and the temperature of the strippingcomposition are selected to provide the desired balance betweenmaximizing removal of the aluminide coating and minimizing base metalattack for a particular coating and metal substrate.

Exposure to the stripping composition causes the aluminide coating onthe surface of the substrate to become degraded. For example, thecoating may have deep cracks, its integrity may be diminished, and itsadhesion to the substrate may be substantially decreased. In someembodiments, the surface may be rinsed by contact with or immersion inwater or an aqueous solution for a short time, e.g., less than about 1minute, to remove the stripping composition and/or degraded coating fromthe surface.

Removal of the degraded coating without damaging the substrate may beaccomplished by various other methods known in the art. For example, thedegraded coating may be removed by abrading the substrate surface, suchas by using a gentle abrasion step that minimizes damage to thesubstrate. As an example, light grit-blasting can be carried out bydirecting a pressurized air stream comprising aluminum oxide particlesacross the surface at a pressure of less than about 40 psi (about 2.8kgf/cm²), typically less than about 20 psi (about 1.4 kgf/cm²). Variousabrasive particles may be used for the grit-blasting, e.g., metal oxideparticles such as alumina, as well as silicon carbide, glass beads,crushed glass, sodium carbonate, and crushed corn cob. The averageparticle size usually is less than about 500 microns, and typically lessthan about 100 microns.

The grit-blasting is carried out for a time period sufficient to removethe degraded coating. The duration of grit-blasting in this embodimentwill depend on various factors. In the case of an aluminide coatinghaving a thickness of from about 50 microns to about 100 microns,grit-blasting will usually be carried out for from about 60 seconds toabout 120 seconds, when utilizing an air pressure of from about 20 psi(about 1.4 kgf/cm²) to about 30 psi (about 2.1 kgf/cm²) and gritparticles having an average particle size of less than about 100microns.

Other known techniques for lightly abrading the surface may be used inlieu of grit-blasting. For example, the surface may be manually scrubbedwith a fiber pad, e.g., a pad with polymeric, metallic or ceramicfibers. Alternatively, the surface may be polished with a flexible wheelor belt in which alumina or silicon carbide particles have beenembedded. Liquid abrasive materials may be used on the wheels or belts.For example, they may be sprayed onto a wheel in a vapor honing process.These alternative techniques can be controlled to maintain a contactforce against the substrate surface that is no greater than the forceused in the gentle grit-blasting technique discussed above.

Other techniques may be employed to remove the degraded material. Oneexample is laser ablation of the surface. Alternatively, the degradedmaterial may be scraped off the surface. In another embodiment, soundwaves (e.g., ultrasonic waves), which may originate from an ultrasonichorn, can be directed against the surface to cause vibrations that canshake loose the degraded material.

In some instances, the degraded coating may be removed by a moreaggressive agitation, e.g., agitation with a force greater than thatproduced using the ultrasonic technique itself. For example, thesubstrate can be immersed in a bath that is rapidly stirred with amechanical stirrer (i.e., for “general agitation”), and that is alsoultrasonically-stirred (i.e., for “local agitation”). Agitation can becarried out until the degraded material is shaken loose. For each ofthese alternative techniques, those skilled in the art would be familiarwith operating adjustments that can be made to control the relevantforce applied to the substrate to minimize damage to the substratesurface.

In some embodiments, an extended rinsing step may be used to remove thedegraded coating without damaging the substrate. This may involvecontacting the degraded aluminide coating with an aqueous solutioncomprising a wetting agent like those described previously, for example,a polyalkylene glycol such as polyethylene glycol. The wetting agent isusually present at a level of from about 0.1% to about 5% by weight,based on the total weight of the rinsing solution. Rinsing can becarried out by a variety of techniques, but is usually undertaken byimmersing the substrate in an agitated bath of the rinsing solution fora time period from about 1 minute to about 30 minutes. The extendedrinsing step can remove chunks of aluminide particles and oxides fromthe substrate. Any remaining thin layer of more coherent aluminidematerial may be removed in another agitation step, or by againcontacting the substrate with the stripping composition.

In other embodiments, the degraded coating may be removed by includingthe step of contacting the degraded coating with a caustic material. Thecaustic may also clean the surface, remove any surface oxides formed asa result of the stripping step, and activate the surface for anyadditional processing steps, such as a second stripping step. Examplesof caustics include potassium hydroxide (KOH), sodium hydroxide (NaOH),ammonium hydroxide (NH₄OH), lithium hydroxide (LiOH), triethylamine((C₂H₅)₃N; TEA), tetramethylammonium hydroxide ((CH₃)₄NOH; TMAH), andmixtures thereof. The contact time can range from about 20 minutes toabout 4 hours, although longer or shorter times may be selecteddepending on the properties of the particular caustic, coating and basemetal.

The caustic may be in the form of a molten salt, but usually is presentas an aqueous solution comprising from about 10% to about 50%, typicallyfrom about 15% to about 30%, more typically from about 17% to about 25%,of caustic, by weight of the composition. The caustic solution usuallyhas a temperature of from about 60° C. to about 100° C., typically fromabout 65° C. to about 90° C., more typically from about 70° C. to about85° C.

The caustic solution may be applied to the substrate in a variety ofways, but as described above, the substrate is typically immersed in abath of the caustic solution. In one embodiment, the substrate islowered into the bath using a suitable rack (for example, one having apolypropylene or other non-conductive surface) that can be raised toremove the substrate after the desired immersion time is reached. Thecaustic solution is typically agitated while in contact with thesubstrate. In one embodiment, this is ultrasonic agitation.Alternatively, a more aggressive agitation, such as described above, maybe used.

A caustic solution such as described above may also be used to pretreat,clean, or remove oxides from the metal-based substrate prior to contactwith the stripping composition. In one embodiment, the caustic solutionis used to clean or remove oxides from the substrate prior to and aftereach contact with a stripping composition. A rinsing step is typicallyprovided between the caustic and acid bath treatments herein to preventpotentially violent reactions between the caustic and acid solutions.

After removal of the coating from the substrate, compressed air may beblown across the substrate to remove any residual aluminide particles,oxides, or abrasive particles. If desired, the substrate can then bere-coated with any suitable material. For example, platinum-aluminideprotective coatings for engine parts can again be applied to the surfaceof a superalloy substrate.

In some embodiments of the invention, the substrate surface may becontacted with two (or more) stripping compositions, in sequence. Thefirst composition may quickly remove some of the aluminide coating. Thesecond (or subsequent) stripping composition may then remove theremaining aluminide coating more slowly, with little or no pitting orattack on the substrate except for the possible occurrence of uniformcorrosion, as discussed previously.

Typically, each stripping composition is present in the form of a bathin which the substrate is immersed. Contact times and bath temperaturesmay vary, as described previously. In one embodiment, the substrate isimmersed in a first bath maintained at a temperature in the range offrom about 4° C. to about 12° C., with an immersion time between about 3and about 4 hours. After rinsing and contact with a caustic solution asdescribed above, the substrate is then immersed in a second bath,typically also maintained at a temperature in the range of from about 4°C. to about 12° C., with an immersion time between about 3 and about 4hours. Additional stripping steps may be used, but are oftenunnecessary. As described above, the substrate can then be subjected tovarious steps to remove the degraded coating.

In another embodiment, the invention comprises a method for selectivelyremoving an aluminide coating from at least one internal surface of ametal-based substrate, comprising the following steps:

-   -   (a) contacting the surface of the substrate with an aqueous        solution comprising from about 10% to about 50%, by weight of        the solution, of caustic at a temperature of from about 60° C.        to about 100° C. for from about 1 to about 4 hours;    -   (b) contacting the surface of the substrate with at least one        stripping composition comprising from about 20% to about 40%        nitric acid, by weight of the composition, at a temperature less        than about 20° C. for at least about 2 hours to degrade the        coating without damaging the substrate;    -   (c) contacting the surface of the substrate with an aqueous        solution comprising from about 10% to about 50%, by weight of        the solution, of caustic at a temperature of from about 60° C.        to about 100° C. for from about 20 minutes to about 2 hours;    -   (d) contacting the surface of the substrate with at least one        stripping composition comprising from about 20% to about 40%        nitric acid, by weight of the composition, at a temperature less        than about 20° C. for at least about 2 hours to degrade the        coating without damaging the substrate; and    -   (e) contacting the surface of the substrate with an aqueous        solution comprising from about 10% to about 50%, by weight of        the solution, of caustic at a temperature of from about 60° C.        to about 100° C. for from about 1 to about 4 hours to remove the        degraded coating without damaging the substrate.

In one embodiment, the caustic solution is ultrasonically agitated whilein contact with the substrate. The caustic solution typically comprisesfrom about 15% to about 30%, more typically from about 17% to about 25%,by weight of the composition, of caustic, such as potassium hydroxide.The temperature of the caustic solution is usually from about 65° C. toabout 90° C., typically from about 70° C. to about 85° C. In oneembodiment, the caustic solution contacts the substrate for a timeranging from about 1.5 to about 2.5 hours in each of steps (a) and (e),and from about 20 minutes to about 1 hour, typically from about 25 toabout 35 minutes, in step (c).

In another embodiment, the method of the invention is used toselectively remove the aluminide coating from the internal shank androot surfaces of a high-pressure turbine blade. The shank portion of theturbine blade typically is a high stress region whose surfaces oftenoperate below the ductile-to-brittle transition temperature of thealuminide coating. This makes the aluminide coating more susceptible tothe formation of minute cracks in the coating that can spread to thesubstrate and lead to a metal fatigue and blade failure. In one aspectof the invention, the aluminide coating is completely removed from theinternal shank surfaces, but not removed from the airfoil internal andexternal surfaces where the protective coating is desired. This can beachieved by immersing the turbine blade in the stripping compositiononly up to the desired level of the shank portion of the blade. Thisavoids the need for masking the airfoil portions of the blade where thealuminide coating is desired. Since the stripping composition is used atrelatively low temperatures, unlike mixtures of hydrochloric acid andnitric acid which are typically heated to increase the stripping rate,acid fumes are not produced that can attack unmasked areas of theairfoil internal and external surfaces.

In another aspect, the invention relates to a turbine engine part havinga metal-based substrate and an aluminide coating on at least one surfacethereof, at least a portion of which coating has been selectivelyremoved from at least one surface of the substrate by a methodcomprising the following steps:

-   -   (a) contacting the surface of the substrate with at least one        stripping composition comprising from about 20% to about 40%        nitric acid, by weight of the composition, at a temperature less        than about 20° C. for at least about 2 hours to degrade the        coating without damaging the substrate; and    -   (b) removing the degraded coating without damaging the        substrate.

In some embodiments, at least a portion of the aluminide coating hasbeen selectively removed from at least one surface of the substrate byusing the above method and selecting various stripping compositions,caustics and processing conditions as described above.

Another aspect of the present invention is directed to a method forreplacing a worn or damaged aluminide coating on at least one surface ofa metal-based substrate, comprising the following steps:

-   -   (i) selectively removing the aluminide coating from the surface        of the substrate by (a) contacting the surface of the substrate        with at least one stripping composition comprising from about        20% to about 40% nitric acid, by weight of the composition, at a        temperature less than about 20° C. for at least about 2 hours to        degrade the coating without damaging the substrate; and (b)        removing the degraded coating without damaging the substrate;        and    -   (ii) applying a new aluminide coating to the surface of the        substrate.

Techniques for applying the new aluminide coating are known in the art.For example, various thermal spray techniques can be employed for thedeposition of overlay coatings. Examples include vacuum plasma spray(VPS), air plasma spray (APS), and high velocity oxy-fuel (HVOF). Otherdeposition techniques can be used as well, such as sputtering andphysical vapor deposition (PVD), e.g., electron beam physical vapordeposition (EB-PVD).

Various techniques are also known for applying diffusion coatings, e.g.,noble metal-aluminide coatings such as platinum-aluminide orpalladium-aluminide. As an example in the case of platinum-aluminide,platinum can initially be electroplated onto the substrate, usingP-salt, Q-salt, or other suitable platinum electroplating solutions. Ina second step, the platinum layer is diffusion-treated with aluminumvapor to form the platinum-aluminide coating.

The following examples illustrate some embodiments of this invention,but should not be construed to be any sort of limitation on its scope.In the examples, each test sample was a high-pressure turbine blade thathad been used for some time in a commercial gas turbine engine. Theturbine blades were made from a single crystal nickel-based superalloy,designated by the trade name René® N5. The turbine blades were coatedexternally with a platinum-aluminide bond coating and an EB-PVDyttria-stabilized zirconia thermal barrier top coat. The internalsurfaces of the turbine blades were coated with a simple aluminidecoating.

EXAMPLE 1

A sample blade was treated according to a process involving multiplesteps. First, the thermal barrier coating was aggressively removed bygrit blasting with aluminum oxide. The blade was then injected with thecommercial acid resistant Plastisol® resin to protect the cooling holesand internal passages from the chemical stripping solution. The bladewas then immersed in a bath formed from a 50:50 (by weight) mixture ofnitric acid and phosphoric acid. The bath was maintained at atemperature of about 170-190° F. (about 77-88° C.). After about 2 to 4hours, the blade was removed and rinsed in cold tap water. ThePlastisol® resin was removed from the internal surfaces by exposing theblade to a temperature of about 1100° F. (about 593° C.) for 1 hour inan air furnace. The external surface of the blade was then lightly gritblasted with 220-mesh aluminum oxide particles at a pressure of about20-30 psi (about 1.4-2.1 kgf/cm²). The above process removed thealuminide coating from the external surfaces of the blade, but not fromthe internal surfaces.

EXAMPLE 2

A sample blade was treated using a process of the invention to removethe aluminide coating from a portion of its internal surfaces withoutdamaging the external platinum-aluminide bond coat. The process involvedpre-treating and activating the surface to be stripped, followed byimmersion of the part in a cold nitric acid solution to degrade thecoating, and then immersion in hot potassium hydroxide solution toremove the degraded coating. First, the root plate from the trailingedge cooling circuit of the blade was removed by grinding to providebetter access to the internal cooling passages of the blade. The bladesurfaces were then pre-treated and activated by immersion in an aqueousbath comprising about 15% to 25% KOH (by weight) maintained at 160-180°F. (about 71-82° C.) for 2 hours with ultrasonic agitation. Afterrinsing in de-ionized water, the blade root and shank were immersed in asolution comprising 30% (by weight) nitric acid and about 0.3% (byweight) Activol® 1658 wetting agent, maintained at 45-55° F. (about7-13° C.), for 3.5 hours to degrade the coating without damaging thesubstrate. After rinsing in de-ionized water, the blade was returned tothe KOH bath for an additional 25-35 minutes, with ultrasonic agitationto remove the degraded coating and re-activate the surface. The bladeroot and shank were again immersed in the cold nitric acid solution for3.5 hours to degrade the remaining coating without damaging thesubstrate. After rinsing in de-ionized water, the blade was returned tothe KOH bath for 2 hours with ultrasonic agitation to remove thedegraded coating.

The above process completely removed the aluminide coating from theinternal and external surfaces of the root and shank portions of theblade with negligible effect on the base metal and on the aluminidecoating on the internal and external surfaces of the airfoil portion ofthe blade. In contrast to Example 1, the process avoided the need formasking the internal cavities of the blade to prevent them from beingblocked by grit particles, and also avoided the use of ahigh-temperature stripping step that can attack the base metal andgenerate acid fumes. The process sequence of alternating cycles ofexposure to hot KOH, rinse, cold nitric acid, rinse, and hot KOH can berepeated a number of times to ensure complete removal of the aluminidecoating from the root and shank portions of the blade.

EXAMPLE 3

A sample blade was treated using another process of the invention toremove the aluminide coating from a portion of its internal surfaceswithout damaging the external platinum-aluminide bond coat. The processinvolved immersion of the part in a cold nitric acid solution to degradethe coating and then immersion in hot potassium hydroxide solution toremove the degraded coating. First, the root plate from the trailingedge cooling circuit of the blade was removed by grinding to providebetter access to the internal cooling passages of the blade. The bladeroot and shank were immersed in a solution comprising 30% nitric acid(by weight) and about 0.3% (by weight) Activol® 1658 wetting agent,maintained at 35-40° F. (about 2-4° C.), for 5 hours to degrade thecoating without damaging the substrate. After rinsing in tap water, theblade was immersed in an aqueous solution comprising about 15% to 25%KOH (by weight) maintained at 160-180° F. (about 71-82° C.) for 2 hourswith ultrasonic agitation to remove the degraded coating.

The above process also completely removed the aluminide coating from theinternal and external surfaces of the root and shank portions of theblade, with negligible effect on the base metal and on the aluminidecoating on the internal and external surfaces of the airfoil portion ofthe blade. These results were obtained even though the process did notuse the KOH pretreatment step used in Example 2, and the nitric acidsolution was maintained at a lower temperature.

Various embodiments of this invention have been described. However, thisdisclosure should not be deemed to be a limitation on the scope of theinvention. Accordingly, various modifications, adaptations, andalternatives may occur to one skilled in the art without departing fromthe spirit and scope of the claimed invention.

1. A method for selectively removing an aluminide coating from at leastone surface of a metal-based substrate, comprising the following stepsin sequence: (a) contacting the surface of the substrate with an aqueoussolution comprising from about 10% to about 50%, by weight of thesolutions of caustic at a temperature of from about 60° C. to about 100°C. for from about 20 minutes to about 4 hours. (b) contacting thesurface of the substrate with at least one stripping compositioncomprising from about 20% to about 40% nitric acid, by weight of thecomposition, at a temperature less than about 20° C. for at least about2 hours to degrade the coating without damaging the substrate; and (c)removing the degraded coating without damaging the substrate.
 2. Themethod of claim 1, wherein the stripping composition comprises fromabout 25% to about 35% nitric acid, by weight of the composition.
 3. Themethod of claim 2, wherein the stripping composition comprises fromabout 28% to about 32% nitric acid, by weight of the composition.
 4. Themethod of claim 1, wherein the stripping composition has a temperatureof from about 0° C. to about 15° C.
 5. The method of claim 4, whereinthe stripping composition has a temperature of from about 4° C. to about12° C.
 6. The method of claim 1, wherein the stripping compositioncontacts the substrate for a period of from about 4 hours to about 10hours.
 7. The method of claim 6, wherein the stripping compositioncontacts the substrate for a period of from about 6 hours to about 8hours.
 8. The method of claim 6, wherein the stripping compositioncomprises from about 25% to about 35% nitric acid, by weight of thecomposition, and has a temperature of from about 4° C. to about 12° C.9. The method of claim 1, wherein the stripping composition furthercomprises from about 0.1% to about 5%, by weight of the composition, ofa wetting agent.
 10. The method of claim 8, wherein the strippingcomposition further comprises from about 01.% to about 5%, by weight ofthe composition, of a wetting agent selected from the group consistingof polyalkylene glycols, glycerol, fatty acids, soaps, emulsifiers, andsurfactants.
 11. The method of claim 10, wherein the substrate isimmersed in a bath of the stripping composition in step (b) for a periodof from about 6 hours to about 8 hours.
 12. The method of claim 1,wherein the substrate is immersed in a bath of the stripping compositionin step (b).
 13. The method of claim 12, wherein the bath is maintainedat a temperature of from about 0° C. to about 15° C. while the substrateis immersed therein for a period of from about 4 hours to about 10hours.
 14. A method for selectively removing an aluminide coating fromat least one internal surface of a metal-based substrate, comprising thefollowing steps in sequence: (a) contacting the surface of the substratewith an aqueous solution comprising from about 10% to about 50%, byweight of the solution, of caustic at a temperature of from about 60° C.to about 100° C. for from about 1 to about 4 hours; (b) contacting thesurface of the substrate with at least one stripping compositioncomprising from about 20% to about 40% nitric acid, by weight of thecomposition, at a temperature less than about 20° C. for at least about2 hours to degrade the coating without damaging the substrate; (c)contacting the surface of the substrate with an aqueous solutioncomprising from about 10% to about 50% a, by weight of the solution, ofcaustic at a temperature of from about 60° C. to about 100° C. for fromabout 20 minutes to about 2 hours; (d) contacting the surface of thesubstrate with at least one stripping composition comprising from about20% to about 40% nitric acid, by weight of the composition, at atemperature less than about 20° C. for at least about 2 hours to degradethe coating without damaging the substrate; and (e) contacting thesurface of the substrate with an aqueous solution comprising from about10% to about 500/o by weight of the solution, of caustic at atemperature of from about 60° C. to about 100° C. for from about 1 toabout 4 hours to remove the degraded coating without damaging thesubstrate.
 15. The method of claim 14, wherein the substrate is immersedin a bath of the stripping composition maintained at a temperature offrom about 0° C. to about 15° C. in each of steps (b) and (d).
 16. Themethod of claim 15, wherein the stripping composition comprises fromabout 25% to about 35%, by weight, of nitric acid.
 17. The method ofclaim 16, wherein the substrate is immersed in the bath of the strippingcomposition having a temperature of from about 4° C. to about 12° C. forfrom about 3 to about 4 hours in each of steps (b) and (d).
 18. Themethod of claim 17, wherein the substrate is immersed in a bath of thecaustic solution having a temperature of from about 70° C. to about 85°C. in each of steps (a), (c) and (e).
 19. The method of claim 18,wherein the caustic solution comprises from about 15% to about 30%, byweight, of potassium hydroxide.
 20. The method of claim 19, wherein thecaustic solution is ultrasonically agitated while in contact with thesubstrate.
 21. The method of claim 20, wherein the aluminide coating isselectively removed from the internal surfaces of a high-pressureturbine blade.
 22. A turbine engine part having a metal-based substrateand an aluminide coating on at least one surface thereof, at least aportion of which coating has been selectively removed from at least onesurface of the substrate by a method comprising the following steps: (a)contacting the surface of the substrate with at least one strippingcomposition comprising from about 20% to about 40% nitric acid, byweight of the composition, at a temperature less than about 20° C. forat least about 2 hours to degrade the coating without damaging thesubstrate; and (b) removing the degraded coating without damaging thesubstrate.
 23. The turbine engine part of claim 22, wherein thealuminide coating comprises at least one compound selected from thegroup consisting of aluminide, platinum aluminide, nickel aluminide,platinum-nickel aluminide, refractory-doped aluminides, and alloyscomprising at least one of the foregoing.
 24. The turbine engine part ofclaim 23, wherein the metal-based substrate is a nickel-based superalloyor a cobalt-based superalloy.
 25. A method for selectively removing analuminide coating from at least one internal surface of a superalloysubstrate, comprising the following steps in sequence: (a) contactingthe surface of the substrate with an aqueous solution comprising fromabout 10% to about 50%, by weight of the solution, of caustic at atemperature of from about 60° C. to about 100° C. for from about 20minutes to about 4 hours. (b) contacting the surface of the substratewith at least one stripping composition comprising from about 25% toabout 35% nitric acid, by weight of the composition, at a temperature offrom about 0° C. to about 15° C. for from about 4 hours to about 10hours to degrade the coating without damaging the substrate; and (c)removing the degraded coating without damaging the substrate.
 26. Themethod of claim 25, wherein the stripping composition comprises fromabout 28% to about 32% nitric acid, and the substrate is immersed in abath of the stripping composition having a temperature of from about 4°C. to about 12° C. for from about 6 hours to about 8 hours.
 27. A methodfor replacing a worn or damaged aluminide coating on at least onesurface of a metal-based substrate, comprising the following steps insequence: (i) selectively removing the aluminide coating from thesurface of the substrate by (a) contacting the surface of the substratewith an aqueous solution comprising from about 10% to about 50%, byweight of the solution, of caustic at a temperature of from about 60° C.to about 100° C. for from about 20 minutes to about 4 hours: (b)contacting the surface of the substrate with at least one strippingcomposition comprising from about 20% to about 40% nitric acid, byweight of the composition, at a temperature of less than about 20° C.for at least about 2 hours to degrade the coating without damaging thesubstrate; and (c) removing the degraded coating without damaging thesubstrate; and (ii) applying a new aluminide coating to the surface ofthe substrate.
 28. The method of claim 27, wherein the strippingcomposition comprises from about 28% to about 32% nitric acid, and thesubstrate is immersed in a bath of the stripping composition having atemperature of from about 4° C. to about 12° C. for from about 6 hoursto about 8 hours.